Typical single shaft, indirectly heated gas turbines comprise a compressor for producing compressed gas, a gas-to-gas heat exchanger for indirectly heating the compressed gas to produce heated compressed gas, an expander for expanding the heated compressed gas, and a generator connected to the single shaft for producing electricity. The control system of single shaft, indirectly heated gas turbines is responsible for safe operation of the power generation plant from start-up to shutdown, and protection against all eventualities. The control system is required to be “fail-safe”, and shut the plant down safely upon the occurrence of any dangerous condition.
In conventional gas turbines the gas is heated by combustion of fuel directly in the compressed gas. This is called Internal Combustion. In an indirectly heated gas turbine, the gas is indirectly heated in a heat exchanger by conduction of heat through membrane walls. The membrane walls are heated by hot gases produced in a separate process, such as external combustion at near atmospheric pressure or such as exhaust gases from an industrial process.
The power of both conventional gas turbines and indirectly heated gas turbines is modulated by control of the temperature of the hot gas entering the expander. In a conventional (internal combustion) gas turbine, modulation of the hot gas entering the expander is achieved by modulation of the combustion of fuel. However, to change the temperature of compressed gas in an indirectly heated gas turbine, an adjustment of the heat input to the heat exchanger, as well as the temperature of the tubes within the heat exchanger needs to change before the temperature of the gas entering the expander will change. The internal components of the heat exchanger, such as the tubes and their supports, heat up and cool down slowly. Thus it is not possible to control the temperature of the heated gas of an indirectly heated gas turbine sufficiently fast for normal power swings, or in an emergency, by control of the heat input to the heat exchanger.
As is normal for all turbo machinery there are two separate control systems to protect against one of the systems developing a fault. The security of two systems is required to protect personnel and the equipment against catastrophic failure of the turbine by overspeed should one of the control systems fail. Two systems are required to meet code, to obtain insurance and avoid onus in litigation in the most unlikely event of failure. These systems must provide sufficiently fast adjustment of the temperature and/or flow of the heated, compressed gas entering the expander to compensate for sudden changes, minor and major, in load, and must also provide a fail-safe means of instantaneous shut off in the event of an overspeed of the turbine.